The 2009 eruption of Redoubt Volcano continues. The last major explosion at Redoubt occurred at 05:55 AKDT April 4. Since then, a lava dome has been growing in the northern part of the summit crater. The shape of the dome when viewed from above is elliptical, with the long axis oriented approximately north-south. A tongue of lava that forms the northern tip of the dome has advanced about 550 yards (500 m) down the Drift Glacier Gorge. The volume of the dome as of early May is approximately 25 - 30 million cubic meters. Renewed explosive activity remains a possibility, and could occur with little or no warning. Such activity would likely result in ash plumes exceeding 30,000 feet (9000 m) above sea level, lahars down the Drift River Valley, and trace to minor ash fall on communities near Redoubt.

Observations

Since the Information Statement of April 11, 2009, AVO field workers have collected new data of several different types, including geologic observations, geodetic measurements, thermal imagery, radar profiles, and satellite views. AVO continues to log seismic data, conduct periodic gas measurements, and make air quality and particulate content measurements. AVO also analyzes all data collected at Redoubt for the purposes of both basic research into volcanic processes and improvements in our forecasting and warning capability.

Observations of the lava dome suggest that it is similar in size and character to the dome that grew between about January 10 and February 15, 1990. Hot blocks shed from the northern edge of the lava dome have accumulated in the Drift Glacier Gorge. This process of rock falls has occasionally generated very minor, localized ash and steam plumes, often visible in the webcam and responsible for occasional dustings of ash on surrounding terrain. A secondary steam-and-ash plume originating about 4000 feet (1220 m) above sea level on the ridge to the west of the Drift Glacier was intermittently visible on AVO's web cameras from about April 5 through April 10. Field observations revealed that this short-lived feature resulted from hot pyroclastic debris, deposited during the April 4 explosion, sifting down into glacial crevasses where it caused minor steam and ash explosions. Observations in the Drift River Valley indicate that the lahar of April 4 was large enough to extend across the entire valley, toppling trees and leaving high-water marks more than 30 feet (10 m) above the active channel.

On April 16, AVO recovered data collected by Global Positioning System (GPS) receivers that had been placed around Redoubt in late February. Retrospective analysis shows that in late March 2009, coincident with the onset of explosive eruption, these GPS instruments recorded a distinct "deflationary" signal. This deflation likely results from the draining of a magma chamber in the mid- to upper-crust. The draining -- or emptying -- of the magma chamber occurs as lava and ash are erupted from the volcano. More than 1 inch (2 cm) of ground movement, directed down and toward the center of the volcano, have been recorded since late March at a station on the east flank of Redoubt. The rate of deformation appears to have slowed starting in early April. In the coming days, AVO will recover additional data from these instruments, which will span the interval from April 16 onward.

Also, on April 16, AVO conducted Forward Looking Infrared (FLIR) measurements of Redoubt's lava dome. The maximum observed temperature was about 420°C (790°F) on the dome's north face. This reading is not a direct measurement of actual lava temperature as it emerges from the vent, which is known to be considerably higher (~850 - 950 °C), but rather a temperature measurement of the cooling crust of new rock that surrounds the extruding magma. Along with providing spot temperature readings, FLIR data also allows AVO to estimate the dimensions of the growing lava dome by clearly delineating the very hot, new rock from the cool background. These measurements, combined with data from RADARSAT2, result in estimated dome dimensions on April 16 of about 1650 ft (500 m) by 2300 ft (700 m) and 500 ft (150 m) thick. Note that recently collected satellite data as well as web cam views show that the dome has grown substantially in the two weeks since these FLIR data were acquired. The volume of the dome and northward flowing lava tongue are now about 25 to 30 million cubic meters.

Seismic activity since April 4 has consisted of numerous shallow events of a distinctly volcanic character, along with some deeper "rock-breaking" earthquakes. The shallow activity likely occurs within 0.5 to 1.5 miles (1 to 2 km) of the summit crater floor and is thought to arise from the ongoing flow of magma into the growing lava dome. The magnitudes of the shallow events range from 0.1 to 1.0, with most being too small to locate. The deeper earthquakes generally range in depth from 3 to 6 miles (5 to 10 km) below sea level, with magnitudes between from 0.2 to 1.8. These deeper earthquakes are indicative of brittle rock fracture, and are thought to represent a stress adjustment to removal of magma from these depths that is feeding the ongoing eruption. AVO has also detected several deeper events at about 19 miles (31 km) beneath Redoubt. These events suggests that magmatic processes are still active at the base of the Earth's crust -- the uppermost layer of the Earth that extends to about 26 miles (40 km) depth in this part of the world -- and that all portions of the Redoubt magmatic system continue to respond to the current eruptive sequence.

AVO has made three fixed-wing gas observation flights on April 16, April 20, and April 28. Measurements from all flights continue to show elevated levels of volcanic gas emission, though the rate of SO2 emission declined from greater than 10,000 tonnes/day to about 2,500 tonnes/day on April 16. On subsequent flights, however, the SO2 emission rate climbed back to approximately 10,000 tonnes/day. CO2 emissions remained fairly consistent over the interval, with the rate ranging between 10,000 to 15,000 tonnes/day.

During clear to partly cloudy weather, satellite observations from NOAA's AVHRR and GOES satellites have shown persistent areas of elevated surface temperature (thermal anomalies) in the dome area at Redoubt. Also during good weather, AVO's web camera located on the observation hut at 5000 feet (1500 m) above sea level and 7 miles north (11.5 km) of Redoubt's summit shows nighttime incandescence radiating from the dome.

Prognosis and Ongoing Hazards

The longest hiatus between explosions during the 1989-1990 eruption was 36 days. In comparison, as of this writing, 28 days have elapsed since the April 4 explosion. Then, as now, dome growth continued through the hiatus, and while dome growth is ongoing, a dome collapse or failure is very likely. If this occurs, an explosive and/or ash-producing event will soon follow, originating from either the vent region at the summit, the disintegrating dome remnants as they tumble down the Drift Glacier Gorge, or from a combination of both. Therefore, AVO considers additional explosive events likely. These events will, as before, produce ash columns tens of thousands of feet tall, with the potential for ash fall on nearby Alaska communities and the possible disruption of air traffic. The lahar danger still exists, particularly immediately following dome failures, but also to a lesser extent during times of quiescence. Between ash-producing events, residents and pilots may also notice an occasional sulfur smell and hazy conditions due to low-level ash and volcanic gas emissions. There is also a possibility that the current episode of dome growth will slowly diminish and no further explosive activity will occur. Based on our knowledge of past eruptions at Redoubt, AVO considers this outcome to be less likely.

Status of Instrumentation

Monitoring capabilities at Redoubt remain strong. Two seismometers and a continuous Global Positioning System (GPS) receiver that were damaged by lightning discharge during the March 23 explosion were repaired on April 16. The real-time network consists of 9 seismometers, a pressure sensor, and 2 web cameras. The Hut camera has a remotely controllable zoom lens that allows close-up imaging of the lava dome. There are 4 additional seismometers, 3 additional GPS receivers, and 1 time-lapse camera that record and store data on site. One of these GPS sites will be upgraded to a continuous telemetered site in the next few weeks. Work at this site and collection of data from instruments with on-site recording will occur as weather and safety conditions permit.

The Alaska Volcano Observatory is a cooperative program of the U.S. Geological Survey, the University of Alaska Fairbanks Geophysical Institute, and the Alaska Division of Geological and Geophysical Surveys.

VOLCANO ALERT LEVELS

NORMALVolcano is in typical background, noneruptive state or, after a change from a higher level, volcanic activity has ceased and volcano has returned to noneruptive background state.

ADVISORYVolcano is exhibiting signs of elevated unrest above known background level or, after a change from a higher level, volcanic activity has decreased significantly but continues to be closely monitored for possible renewed increase.

WATCHVolcano is exhibiting heightened or escalating unrest with increased potential of eruption, timeframe uncertain, OR eruption is underway but poses limited hazards.

WARNINGHazardous eruption is imminent, underway, or suspected.

AVIATION COLOR CODES

GREENVolcano is in typical background, noneruptive state or, after a change from a higher level, volcanic activity has ceased and volcano has returned to noneruptive background state.

YELLOWVolcano is exhibiting signs of elevated unrest above known background level or, after a change from a higher level, volcanic activity has decreased significantly but continues to be closely monitored for possible renewed increase.

ORANGEVolcano is exhibiting heightened or escalating unrest with increased potential of eruption, timeframe uncertain, OR eruption is underway with no or minor volcanic-ash emissions [ash-plume height specified, if possible].

REDEruption is imminent with significant emission of volcanic ash into the atmosphere likely OR eruption is underway or suspected with significant emission of volcanic ash into the atmosphere [ash-plume height specified, if possible].